There are players (multibillionairs several times over, usually) who would drop a load of e.g. Platinum for a new player. There are also pirates (ok, nothing much to get here) and worse scum, although I haven't heard yet of them baiting newbies this way.
If you can afford the rebuy on a Hauler or an Adder without too much headache, there's not much to loose. If you meet up inside the no-fire zone of a station (
not in an Anarchy system, though), they'll get their behinds thoroughly paddled if they try anything kinky. Those station guns are not something you'd want to be in front of...
As for their reasons - once you can just walk into any station and get any ship you want by snapping your fingers, and it doesn't even register on your account, there's pretty little more except for exploration or social interaction. And giving a new player a leg up will increase the chance of them sticking around until they reach the bend in the learning curve, therefore more players available to play with - or against.
Worst case, you're out ~5000 Cr rebuy for a
stock Adder with a full cargo loadout and gained a tale to tell here (no names, though - for that, you'll have to find a combat wing).
On the other hand, if you want to RP your way through your Galaxy, this might be a good reason not to accept such an offer.
As for this being common - depends on the circumstances. On rare CGs, the Hutton Truckers usually show up with a bunch of large trading ships, offering to load up anyone interested with rares for free. I can just guess at their reasons, but probably simply to get actual people into the instance instead of simply NPCs. In BH or CZ CGs, you can usually find a wing of experienced pilots who will let you (or offer you to) wing up with, sharing the profits.
Speaking for myself - we want to keep CMDRs active in this game. Killing them off while they're young (a.k.a. "Sealclubbing") doesn't help there. Once you are grown up, though, things might be a little bit different
